Oil recovery in producing wells is oftentimes impeded because of a tendency for the casing perforations through which oil passes into the well annulus to become clogged or otherwise damaged. This problem can be remedied to a certain extend by treating the perforations with a highly caustic liquid treating fluid. This fluid is introduced into the well annulus adjacent to the perforations by a pipe string extending from the surface of the well through which the treating fluid is passed. Since perforations occur at various different depths in different wells, or even in the same well, treatment may take place at a certain level and then it becomes necessary to add further pipe sections to the pipe string to effect treatment at lower levels. During the connection or disconnection of such pipes, it is essential that the fluid be retained in the pipe string not only from a safety standpoint but simply to avoid wasting the treating fluid.
With the above in mind, it is the practice to introduce into the pipe string an appropriate control valve which might constitute an entire sub section simply substituted for one of the pipe sections in the string. Basically, the control valve must be operable in response to a predetermined application of hydraulic force in order that the same can be controlled from the surface. Accordingly, conventional valves of which I am aware all incorporate a valve seat and ball seated on the downstream side of the valve seat. The ball in turn is biased into closed condition on the valve seat by an appropriate compression spring.
With the foregoing arrangement, it is only necessary to provide a hydraulic force on the ball to oppose the force of the spring and thereby unseat the ball and open the valve. The caustic treating fluid can then pass through the valve to the well annulus to treat the perforations.
One problem with the foregoing types of valves is the fact that should the spring fail so that the ball is not held on the seat, the valve fails open, and thus all fluid will drain. Another problem is the fact that once the hydraulic pressure of the treating fluid is increased sufficiently to overcome the spring pressure and thereby unseat the valve, initial flow of fluid through the valve will result in a drop in pressure across the valve. The spring will then reseat the valve and pressure will build up and then unseat the ball and a "chattering" situation can exist.
In addition to the foregoing, many prior art control valves for the purposes set forth constitute entire sub assemblies which are to be substituted within the pipe string for one of the pipe sections. Should the valve sub body be of larger diameter than the pipe string, it constitutes a hazard in possibly causing a "hang-up".